1. Field of the Invention
The present invention relates to a method and device for monitoring the instantaneous behavior of a tyre during the running of a motor vehicle.
2. Description of the Related Art
A tyre of a motor vehicle, when rolling on a road surface, is subject to a vertical load and to longitudinal and transverse external forces which are generated in the various manoeuvres, such as braking, acceleration, cornering, etc.
During each manoeuvre, any point on the tyre rolling in steady state and in transitory conditions carries out a movement which can be represented by displacements in the three spatial directions (Cartesian co-ordinates) for each cycle of the revolution of the tyre. The movement of the point has characteristics which are typical of the position of each particular point of the tyre, of the structure and of the external profile of the tyre, of the conditions of use (velocity, load and pressure), of the conditions of the surface on which the tyre rolls, and of the external forces to which the tyre is subjected in the manoeuvres of braking, acceleration, cornering, etc.
The inventors have perceived that the variation of the displacements in space of a specified point, located on the rolling tyre and subject to specific forces, has the same basic shape (basic curve) for each revolution of the tyre, while it varies as to the position and amplitude of the peaks, and/or the start and end of each peak, and the relative maxima and/or minima. Similarly, the variation of velocity in the space of the point has the same basic shape for each revolution of the tyre.
The inventors have identified a set of basic reference curves which represent cyclic reference displacements in space, and the relative velocities for each specified point of the tyre in specified running conditions. More particularly, the basic reference curves are acquired and stored either during the running of the motor vehicle on the road or in the course of standard tests or by means of calculation. The inventors have found that these basic reference curves can advantageously be used to make comparisons of various kinds with cyclic operating curves and to obtain, from these comparisons, information on the behaviour of the tyre when rolling, on the forces exchanged, on the surface on which it is rolling, on the conditions of use, on the circumstances of adhesion (friction) available at each moment between the tyre and the road, etc., in the instant in question. In particular, it is possible to ascertain the way in which the displacement (or velocity) of the point concerned varies with time, when moving from perfect adhesion to total slip of the tyre.
The information acquired by means of the comparison can be used to set interventions on the mechanisms of the motor vehicle and to monitor its behaviour, for example during braking, acceleration, drift, and similar.
In the case of the present description and the following claims, the significance of the term xe2x80x9ccontinuouslyxe2x80x9d, relating to the phases of signal acquisition, signal comparison and similar, also covers acquisition and comparison carried out at times separated from each other by brief intervals, where the duration of these intervals is sufficiently brief to enable the behaviour of the tyre in the running conditions in question to be represented in a significant way. Typically, the maximum duration of such an interval is equal to the time required for a tyre to complete a whole revolution of 360xc2x0 at the velocity in question.
In a first aspect, the invention relates to a method for monitoring the instantaneous behaviour of a tyre during the running of a motor vehicle, the said tyre comprising at least one tread and one casing, three spatial directionsxe2x80x94one longitudinal, one transverse and one verticalxe2x80x94being associated with the said tyre, and the said method comprising the steps of
a) acquiring and storing, at least temporarily, at least one basic reference curve which represents the variation of the displacement of at least one specified point of the said tyre in at least one of the said three spatial directions, as a function of the spatial position of the said point, in at least one portion of a revolution of the said tyre,
b) continuously acquiring signals of the position in space of the said at least one point of the said tyre, in at least one portion of a revolution of the said tyre,
c) deriving from the said position signals at least one cyclic curve of current operating displacement of the said at least one point in the said at least one of three specified spatial directions, as a function of the spatial position of the said point, in at least the said portion of a revolution of the said tyre,
d) continuously comparing the said curve of current operating displacement with the said basic reference curve of displacement which is stored, and
e) emitting a signal depending on the said comparison, the said signal indicating the said instantaneous behaviour of the said tyre.
In one embodiment, the said basic reference curve of displacement mentioned in point a) above is acquired during the running of the motor vehicle on the road, in the course of one revolution of the tyre, and is stored temporarily, for a specified period, to make the said comparison, as mentioned in point d) above, with the said curve of operating displacement acquired in a subsequent revolution of the said tyre.
Typically, the said basic reference curve of displacement which is stored is that of the immediately preceding revolution.
In another embodiment, the said basic reference curve of displacement mentioned in point a) above is acquired in a step which precedes the step of the operation of the said motor vehicle and is stored permanently in order to make the said comparison, as mentioned in point d) above, with the said curve of operating displacement acquired in the said at least one portion of a revolution of the said tyre.
Preferably, a basic reference curve of velocity of the said at least one point in one spatial direction is stored in the said step a), and a cyclic curve of operating velocity of the said at least one point in one spatial direction is determined in the said step c), the said curves of velocity being obtained, respectively, from the said basic reference curve of displacement and from the said curve of operating displacement by means of a mathematical derivative with respect to time, the said basic reference curve of velocity and the said curve of operating velocity being compared in the said step d).
Advantageously, the state of deformation of the said casing in the vicinity of one of its points in an area of contact between the tyre and the road is measured by means of the said basic reference curve and the operating curve of displacement or velocity, to evaluate the adhesion present between the tyre and the road, and the capacity of the said tyre to develop forces tangential to the contact with the said road.
Preferably, signals of the positions of at least two points of the said tyre are acquired in the said step b), and the corresponding curves of operating displacement or velocity are also compared with each other in the said step d) to reveal rapid changes in the aforesaid state of deformation or any acquisition errors.
In a second aspect, the invention relates to a device for monitoring the instantaneous behaviour of a tyre during the running of a motor vehicle, the said tyre comprising at least one tread and one casing, three spatial directionsxe2x80x94one longitudinal, one transverse and one verticalxe2x80x94being associated with the said tyre, and the said device comprising
i. means for acquiring and storing, at least temporarily, at least one basic reference curve which represents the variation of the displacement of at least one specified point of the said tyre in at least one of the said three spatial directions, as a function of the spatial position of the said point, in at least one portion of a revolution of the said tyre,
ii. at least one sensor associated with the said at least one point of the said tyre, capable of emitting, over a period of time, signals of the position in space of the said point,
iii. pick-up means capable of continuously acquiring the said signals of the position in space of the said at least one point of the said tyre, in at least the said portion of a revolution of the said tyre, and
iv. processor means incorporating a program capable of finding from the said position signals at least one cyclic curve of current operating displacement of the said at least one point in the said at least one of three specified spatial directions, as a function of the spatial position of the said point, in at least the said portion of a revolution of the said tyre,
v. the said processor means being capable of continuously comparing the said curve of current operating displacement with the said basic reference curve of displacement which is stored,
vi. the said processor means being additionally capable of emitting a signal depending on the said comparison, the said signal indicating the said instantaneous behaviour of the said tyre.
In one embodiment, the said storage means are capable of acquiring and storing temporarily, for a specified time, the said basic reference curve of displacement mentioned in point i) above during the running of the motor vehicle on the road, in the course of one revolution of the tyre, and the said processor means carry out the said comparison mentioned in point v) above between the said temporary basic reference curve of displacement and the said curve of operating displacement, acquired in a subsequent revolution of the said tyre.
In another embodiment, the said storage means are capable of acquiring and storing permanently the said basic reference curve of displacement mentioned in point i) above in a step which precedes the step of operating the said motor vehicle, and the said processor means carry out the said comparison mentioned in point v) above between the said permanently stored basic reference curve of displacement and the said curve of operating displacement acquired in the said at least one portion of a revolution of the said tyre.
Preferably, the said processor means are capable of storing a basic reference curve of velocity of the said at least one point in one spatial direction and are capable of determining a cyclic curve of operating velocity of the said at least one point in one spatial direction, the said curves of velocity being obtained, respectively, from the said basic reference curve of displacement and from the said curve of operating displacement by means of a mathematical derivative with respect to time, the said processor means being capable of comparing the said basic reference curve of velocity with the said curve of operating velocity.
Advantageously, the said processor means measures the state of deformation of the said casing in the vicinity of one of its points in an area of contact between the tyre and the road, by means of the said basic reference curve and the operating curve of displacement or velocity, to evaluate the adhesion present between the tyre and the road, and the capacity of the said tyre to develop forces tangential to the contact with the said road.
Preferably, at least two sensor means are associated with at least two points of the said tyre and emit at least two signals of the position of the said at least two points, and the said processor means determine by means of the said signals at least two curves of operating displacement or velocity, and compare them with each other to reveal rapid changes in the aforesaid state of deformation or any errors in acquisition.
The inventors have observed that, for the purposes of the present invention, the most significant points of the tyre are:
points on the sidewall of the casing, referred to here as points on the casing sidewall level;
points located on the outer surface of the tread of the tyre, referred to here as points at the tread level;
points located on the inner liner of the casing, referred to here as points at the liner level,
intermediate points within the structure of the casing (for example points between the tread and the liner of the casing).
Preferably, the point is specified in such a way that it is located on the equatorial plane of the tyre (centre or crown), or on the shoulders of the tyre.
Typical physical quantities measured by the method and device according to the invention are:
longitudinal displacement of the point (direction X),
transverse (lateral) displacement of the point (direction Y)
vertical displacement (direction Z).
longitudinal velocity of the point (direction X),
transverse (lateral) velocity of the point (direction Y)
vertical velocity (direction Z).
The variation of the displacements or velocities of the point with time can be used to find, from time to time, the current (instantaneous) operating situation of the tyre, for example the state of instantaneous stressing of the tyre and the available adhesion between the tyre and the road.
One of the distinctive characteristics of the method and device according to the invention lies in the fact that the signals originate directly from the tyre, which acts as a sensor, and are not mediated by other mechanical elements, such as a rim or hub of the wheel on which the tyre is fitted. This provides the advantage of avoiding the delay times and potential interference typical of conventional devices to prevent locking in braking (antiskid or ABS) and of conventional devices to prevent slip in acceleration (antislip).
Another distinctive characteristic of the method and device according to the invention lies in the fact that they make it possible to monitor the behaviour of the tyre in braking, acceleration and cornering (drift), owing to the monitoring of the variation with time of the state of longitudinal and lateral deformation of the points of the tyre casing within the footprint.
This capacity to monitor the behaviour of the motor vehicle when cornering, by determining the behaviour of the tyre in the drift condition, is one of the salient aspects of the present invention. This is because, as far as the inventors are aware, none of the known methods and devices has this important characteristic.
In the course of the present description and of the claims, the term xe2x80x9cbasic curvexe2x80x9d, also referred to as a xe2x80x9ccyclic curvexe2x80x9d, denotes a curve which represents the variation of either the displacement or the velocity of one point of a tyre in a specified spatial direction as a function of the spatial position of the point, in a revolution of the tyre or in a portion of a revolution, in the conditions under consideration.